In many types of turbines, a cover is provided adjacent the tips of the buckets, the cover rotating with the buckets and in close proximity to a stator segment. The bucket covers may be mounted individually on each bucket tip or span a plurality of bucket tips. Leakage paths between the bucket covers and the stator segment reduce turbine performance and accordingly, various devices have been proposed and used to minimize such leakage paths. Ideally, brush type seals theoretically provide the best known seals. However, a brush seal at this location presently appears impractical because of the high surface speeds of the covers relative to the stator segment and potential brush bristle wear.
Labyrinth type seals have been proposed and used in the past to effect a seal at these locations. In one such arrangement, axially spaced, radially inwardly projecting teeth are formed on the stator segment and lie in radial opposition to a plurality of lands formed on the radially outer surface of the bucket cover. This is a three land concept known as a "W" cover seal.
Another type of seal provides for a plurality of axially spaced, radially inwardly projecting labyrinth teeth formed on the stator segment with the end teeth projecting radially inwardly further than the teeth between the end teeth. On the bucket, the cover has a central depression and a through opening. A pin internal to the bucket extends through the opening and is peened to retain the bucket cover on the bucket. The cover has lands between the end teeth of the stator segment adjacent the tips of the reduced length teeth. While this arrangement has been satisfactory, there remained the possibility of further reducing the leakage path without increasing the risk for performance deterioration due to rubbing.
Additionally, vernier type seals with labyrinth teeth in radial opposition to one another on the bucket cover and stator segment are known. See, for example, the paper entitled "Thirty Years Experience With Integrally Shrouded Blades", ASME Power Division 1989.